The present invention relates to acquiring a satellite observation image of the earth by push-broom scanning using one or more strips (or matrix(ces)) of detectors of the charge coupled device (CCD) type travelling relative to the observed zone.
The principle of push-broom type scanning is shown in FIG. 1 for the case of a single strip 1 of detectors.
As the satellite carrying the strip 1 travels, the strip observes successive lines L1, L2, . . . , Ln perpendicular to the travel direction (arrow D). At each instant, an optical system 2 of the instrumentation forms the image of a line of landscape on the line of detectors, with the strip 1 being placed in the focal plane of the optical system 2, perpendicularly to the speed vector of the satellite. The landscape scans over each detector which integrates light flux over an exposure time and transforms it into a proportional electric charge.
FIG. 2 shows a conventional system for processing images taken in this way.
That processing system comprises, in outline: a unit 3 for processing and amplifying the output from the detectors of the strip 1; an analog-to-digital encoder 4 that receives the signal output by the unit 3; transmitter means 5 for taking the digital images picked up in this way and transmitting them from the satellite to the ground; and a unit 6 on the ground for reconstituting the images.
The unit 3 includes, in particular, a shift register into which the information as integrated and stored in charge form in each detector of the strip 1 is transferred at the end of an exposure time. Thereafter, the register transfers the charges in the form of electrons, and the charges are converted into a succession of voltages that are proportional to the received and integrated light fluxes.
The unit 6 on the ground reconstructs the images, in particular by implementing deconvolution processing to compensate for instrument defects, and where appropriate interpolation processing to reconstitute certain pixels within the image.
It is known that the finer the resolution of an image, the more the signal-to-noise (SNR) tends to diminish, in particular because of the lack of light flux, so that images cease to be acceptable.
Unfortunately, presently known solutions for mitigating that drawback are not satisfactory.
In particular, one possible solution consists in increasing the dimensions of the instrument, and in particular the diameter of the pupil of the telescope. However that solution is expensive.
Another solution consists in controlling the attitude of the satellite so as to slow down its rate of scanning. That technique allows light flux to be integrated for longer, but it implies a loss of continuity in the track of the satellite, and consequently leads to a loss of data.
A third solution consists in using special time delay integration (TDI) detectors. These detectors are constituted as matrices in which the rows are shifted electronically so as to compensate for the speed of the satellite. Nevertheless, such a system is complex to embody and implement. Furthermore, it does not enable sampling to be performed that is adapted to the modulation transfer function so as to ensure that spectrum folding is negligible and acquisition data rate is optimized.
An object of the invention is thus to provide an acquisition method of this type in which instrument noise is reduced and which does not suffer from the drawbacks of the above-mentioned solutions.
Theoretical Background
It is known that image acquisition by push-broom scanning can be modelled linearly by the formulation:
Ib=Πp(h{circle around (xc3x97)}◯+b)
where:
{circle around (xc3x97)} designates the convolution operation;
◯ is the landscape whose image is to be acquired;
h is the impulse response of the instrument;
b is the noise superposed on the filtered landscape;
Ib bis the raw image; and
Πp is a two-dimensional Dirac comb which means that the continuous image has been made discrete.
The image is digital and encoded on a limited number of bits.
A spectral representation is often preferred. It is represented by the Fourier transform (FT) of the raw image, i.e.:
Îb=Πp{circle around (xc3x97)}(MTF.Ô+b)
where the hats indicate that these are Fourier transforms, and where the abbreviation MTF stands for modulation transfer function, which is the Fourier transform of the impulse response. In the spectral representation, the convolution operation is equivalent to a multiplicative operation.
The parameters that are mainly involved in the above equations are described in greater detail below.
Modulation Transfer Function or MTF
The MTF is the attenuation factor of spatial frequencies. The higher the frequency the lower the MTF. The geometrical location where the MTF becomes zero is the boundary of the instrumental cutoff. The instrument spectral medium is the low frequency domain as limited by the instrumental cutoff boundary.
The MTF of an instrument is the product of the MTF of its optical system multiplied by the MTF of its detector and by the MTF of its displacement in the travel direction.
Noise
Instrumental noise is always present. A simplified approach consists in considering that it is characterized by the mean SNR (generally known once the mission of the satellite has been defined), and that, compared with other noise, Poisson type photon noise is preponderant at the mean luminance. With such a distribution, the standard deviation of the noise varies with the square root of the number of photons N picked up by the detector. Consequently:
SNR={square root over (N)}
Sampling Array
Present satellites generate a square orthogonal array (speed and strip are in orthogonal directions). The sampling step size in the speed direction on the ground is xcex94xcfx84=v.te (where v is the speed of the satellite and te is the sampling time), and the sampling step size in the strip direction is equal to the distance between two adjacent elementary CCDs. Consequently, using the arrays implemented in present satellites, the two pitches are equal and the projections of ground pixels do not overlap (the ground is scanned once only).
However, with acquisition as performed by current satellites, the sampling frequency is equal to the cutoff frequency so Shannon""s condition is not satisfied (te less than xc2xdfc, where fc is the cutoff frequency). This results in a high level of spectrum folding that gives rise to artifacts and makes any attempt at deconvolution or interpolation difficult.
To mitigate that drawback, proposals have recently been made, in particular in French patent FR 2 678 460 in the name of the Applicant, to use a technique whereby two offset strips make it possible to perform oversampling so as to limit spectrum folding.
In its patent application WO 97/05451, the Applicant has also proposed techniques for implementing staggered oversampling adapted to the modulation transfer function of the instrument so as to enable the amount of spectrum folding to be negligible while nevertheless optimizing acquisition data rate.
The invention proposes an instrumental solution which makes it possible to obtain images in which firstly spectrum folding is limited and secondly noise is minimized.
More particularly, the invention provides a method of acquiring and processing a satellite observation image of the earth by means of at least one strip or matrix of detectors of the charge coupled device type, said detectors travelling over the observed zone, in which a plurality of pixels are acquired in a given sampling, the method being characterized in that that array of said sampling includes xe2x80x9cexe2x80x9d subarrays corresponding to sampling satisfying Shannon""s condition, where xe2x80x9cexe2x80x9d is an integer or rational number greater than 1, and in that processing is implemented that enables a noise-reduced image to be interpolated at a resolution corresponding to that of a sampling subarray on the basis of the initially acquired image pixels.
In other words, the initial image is equivalent to xe2x80x9ceexe2x80x9d images corresponding to sampling that satisfies Shannon""s condition, with which a new image at the same resolution is reconstituted. For each pixel of the new image, xe2x80x9cexe2x80x9d times more flux is available than in the case of normal acquisition, such that the mean noise in the new image is divided by {square root over (e)} compared with the noise normally obtained for an image of the same resolution.
The invention also provides a camera instrument for implementing the method.